Digital Signal Processing (DSP) techniques enable to compensate impairments affecting optical signal during transmission over optical fiber by applying inverse filter properties of the impairments. These techniques enable the transmission of higher rate channels on longer reach and can be applied at the receiver receiving an optical signal through a medium, such as optical fiber. Notably, coherent reception technique enables to get the information on both phase and amplitude of the received signal. In this way, the DSP compensates for impairments caused during transmission, before reception, by applying appropriately calculated filters.
The benefits of digital signal processing are not limited to the application of this technique to the receiver end. Comparably, DSP techniques combined with Digital to Analog Converter (DAC) can be applied at the transmitter side. In such a case, the transmitter, hereafter called digital transmitter, including a DSP processor and a DAC to convert digital signal to analog signals are used to drive an optical modulator.
The DSP in a digital transmitter can be also used to mitigate at the transmitter side nonlinear impairments appearing in the transmission inside of the optical fiber, as illustrated by NPL (non-patent literature) 1. Such DSP technique for processing signals at the digital transmitter in order to compensate or mitigate impairments appearing during transmission is called pre-compensation or pre-distortion depending on sources.
Modulation in a digital transmitter can be performed with an optical IQ (In phase/Quadrature phase) modulator, sometimes called Cartesian modulator, vector modulator, dual parallel modulator or nested modulator depending on the sources. In an IQ modulator, electric signals drive two independent Mach-Zehnder devices which are called children Mach-Zehnder Modulators (MZM), and which modulate the phase and the amplitude of the same optical carrier and whose outputs are relatively phase delayed by 90 degrees before being recombined. These components are called Inphase (I) and Quadrature phase (Q) of the signal. The phase difference between the outputs of the children MZM can be called the angle of quadrature and is ideally 90 degrees. Such IQ modulators are also widely used in optical transmitters.
Additionally, relevant to the present application, PTL (patent literature) 1, PTL2, PTL3, PTL4 and NLP2 disclose bias control circuits for MZMs.